148 FISHES CHAP. 



invariably present. The olfactory organs are obviously paired, and 

 they are distinct from the hypophysis, Paired limbs are present. 



As previously stated, the true Fishes form the second of the 

 six " classes " into which the Craniata are divided. As compared 

 with the higher Craniata, their distinctive characters may be 

 concisely stated as follows : — 



Fresh water or marine Gnathostomata, which in their shape 

 and in method of breathing are adapted for an aquatic life. 

 Throughout life their respiratory organs are in the form of 

 vascular processes (gills) derived from the walls of the branchial 

 clefts, and supported by a series of branchial arches. The 

 principal organ of locomotion is the powerful muscular tail ; in 

 addition, however, there are paired fins, pectoral and pelvic, 

 corresponding to the fore- and hind-limbs of the terrestrial 

 Craniata, and possessing a supporting cartilaginous or bony 

 skeleton (" ichthyopterygium ") which cannot readily be com- 

 pared with the limb-skeleton of the latter. Fishes also possess 

 a system of median fins, supported by a special skeleton of their 

 own. An exoskeleton of dermal spines or denticles, scales or bony 

 plates, is usually present. Except in one group, the Dipnoi, the 

 heart has but one auricle, and receives only venous blood, which 

 it forces, first, through the blood-vessels of the gills, and thence, 

 as arterial blood, through the vessels of the body generally. An 

 air-bladder is frequently present, and serves as a hydrostatic organ 

 or float, but in a few cases it may act as a lung, and helps the 

 gills in the work of respiration. The paired olfactory organs rarely 

 communicate with the oral cavity by internal nostrils. Peculiar 

 cutaneous sense-organs are disposed in linear tracts along the sides 

 of the body (lateral line sensory organs), and on the head, and 

 appear to be specially associated with a life in water. 



Fishes may be divided into the following " sub-classes," and 

 these in turn may be subdivided into various " orders " and 

 " sub-orders " : — 



(i.) Blasmobranchii ; e.g. Sharks, Dog-Fislie3, Skates, and Eays. 



(1) Pleuropterygii t ; e.g. Cladoselache. 



(2) Iclithyotomi j ; e.g. Pleuracanthus. 



(3) Acanthodei f ; e.g. Acanthodes. 



(4) Plagiostomi. 



(a) Selachii ; e.g. many extinct and all living Sharks 



and Dog-Fishes. 



(b) Batoidei ; e.g. Skates and Rays. 



(5) Holocophali ; e.g. Chimaera and Callorhynchus. 



